[4.8.3]What exactly does FString GGameUserSettingsIni contain?

I know how to use the global variables to select which ini to read and write from for setting configs.(GGameUserSettingsIni, GGameIni, ect)

But what if I want to make a completely new and custom ini? I assume the global FString contains the filename and possibly the directory location of the file? If I were to use my own FString, what would it need to contain to properly point to the custom ini file?